1. Field of the Invention
The present invention relates generally to a gain measurement device for an optical amplifier. More particularly, the invention relates to gain measurement device and method for an optical amplifier which can continuously measure gain versus wavelength characteristics of an optical amplifier to be measured at high speed and high precision.
2. Description of the Related Art
Conventionally, in an optical communication, such as a wavelength division multiplexing communication to be used for a large capacity and long distance optical transmission system, for example, level deviations between respective channels (wavelength) cause deterioration of signals. On the other hand, in the long distance transmission, characteristics of the optical amplifiers to be used at appropriate interval are important factor to cause deterioration of signals. Accordingly, in addition to low-noise characteristics and high efficiency are required for the optical amplifier, flattening and widening of band of a gain versus wavelength are required. For this purpose, evaluation of the gain versus wavelength characteristics of the foregoing optical amplifier has been heretofore important.
FIG. 6 is an illustration for explaining the conventional gain measuring system for the optical amplifier of this kind. Referring to FIG. 6, a multiple wavelength light source 10 is designed for outputting saturated lights having a plurality of predetermined wavelengths xcex1 to xcexn. A variable wavelength light source 112 is designed for outputting a fine probe light of variable wavelength. A polarization scrambler 113 is designed to make a light polarization surface for the variable wavelength fine (very little) probe light to output a polarized light to an optical coupler 11. The optical coupler 11 multiplexes the output from the multiple wavelength light source 10 and the output from the polarization scrambler 113. A variable light attenuator 14 is designed for performing level control of the output from the optical coupler 11 depending upon a control signal from a control portion 21.
An optical switch 15 receives an output of the optical attenuator 14 at an input port 15a to selectively output through one of output ports 15b and 15c. A measurement objective optical amplifier 16 has an input 16a connected to one output port 15c of the optical switch 15 and an output 16b connected to one input port 17b of an optical switch 17 at a next stage. The other output port 15b of the optical switch 15 is connected to an input port 17a of the optical switch 17. The optical switch 17 is designed to arbitrarily establish connection between two input ports 17a and 17b and two output ports 17c and 17d. 
The output port 17c of the optical switch 17 is connected to a light power meter 18. The other output port 17d of the optical switch 17 is connected to an optical spectrum analyzer 19. Outputs of the optical power meter 18 and the optical spectrum analyzer 19 are fed to a gain measuring portion 20. An output of the optical power meter 18 is input a control portion 21.
FIGS. 7 and 8 are operational flowchart for explaining a gain measurement process of the conventional optical amplifier shown in FIG. 6. In FIG. 7, at first, control is performed so that only multiple wavelength lights (xcex1 to xcexn) from the multiple wavelength light source 10 are input to the measurement objective optical amplifier 16. At this condition, the control portion 21 monitors an input power of the measurement objective optical amplifier 16 by the optical power meter 18 to control an attenuation amount (ATT amount) of the variable optical attenuator 14 so that the input power becomes a rated (nominal) value (Pnom) (step S01).
At this condition, by employing the optical spectrum analyzer 19, an input spectrum Pin1(xcex) of the input for the measurement objective optical amplifier 16 is measured (step S102). The input spectrum at this time is shown on left side of FIG. 9A. In the condition where only lights having wavelengths xcex1 to xcexn of the multiple wavelength light source are input, an output spectrum Pout1 of the measurement objective optical amplifier 16 is measured by means of the optical spectrum analyzer 19 (step S103). The output spectrum at this time is shown on right side of FIG. 9A.
Here, as shown in FIG. 10, a relationship between an input and an output of the optical amplifier 16 is expressed by:
Pinxc3x97G+Pase=Pout 
wherein G is a gain, Pase is a power of a spontaneous emission light.
Accordingly, when only lights having wavelengths xcex1 to xcexn of the multiple wavelength light source is input, the relationship between input and output of the optical amplifier 16 is expressed by an expression (1) of FIG. 9A.
Thereafter, the light of the multiple wavelength light source 10 and the fine probe light by the variable wavelength light source 112 are superimposed to be input to the measurement objective optical amplifier 16. Then, the control portion 21 monitors the input power of the amplifier by means of the optical power meter 18 for controlling attenuation amount of the variable optical attenuator 14 in order to maintain the input power at the rated value (step S104). At this time, a wavelength of the probe light emitted from the variable wavelength light source 112 is assumed to be set at xcexxe2x80x21.
At this condition, an input spectrum Pin2 (xcex) of the measurement objective optical amplifier 16 is measured by means of the optical spectrum analyzer 19 (step S105). The input spectrum to the measurement objective optical amplifier 16 at this condition is shown on the left side (solid line) of FIG. 9B. Then, an output spectrum Pout2 (xcex) of the measurement objective optical amplifier 16 is measured by means of the optical spectrum analyzer 19 (step S106). The output spectrum at this time is shown on right side of FIG. 9B.
Next, the wavelength of the variable wavelength light source 112 is varied to xcexxe2x80x22 (see FIG. 9B) to repeat the foregoing process through steps S104 to S106 (step S107). Similarly, for xcexxe2x80x23 to xcexxe2x80x2m, the process through steps S104 to S106 is repeated respectively (steps S108 to S109). Finally, the expression (2) of FIG. 9B can be obtained. A solution of the expressions (1) and (2) is obtained with respect to G(xcex) to establish an expression (3) as shown in FIG. 9C (step S110 of FIG. 8). G(xcex) obtained from the expression (3) is indicative of a gain G(xcexxe2x80x21), . . . , G(xcexxe2x80x2m) of wavelength (xcexxe2x80x21 to xcexxe2x80x2m) of the variable wavelength light source. On the other hand, gains G (xcex1), . . . , G (xcexn) of wavelength xcex1 to xcexn of the multiple wavelength light source obtained from the expression (1) of FIG. 9A obtained from the input and output spectrum in the process through steps S101 to S103, namely, in the condition where only the light from the multiple wavelength light source 10 is input, are obtained arithmetically (step S111).
From these steps S110 to S111, gain versus wavelength characteristics in full wavelength band of the measurement objective optical amplifier 16 can be derived (step S112). One example of the result of measurement is shown in FIG. 11.
In the conventional gain measurement method of the foregoing optical amplifier, it becomes necessary to repeat processes for setting wavelength of the variable wavelength light source 112, setting of the optical spectrum analyzer 19 and so forth per wavelength to be measured. Accordingly, when number of wavelengths to be measures is increased, the measurement period is proportionally expanded.
On the other hand, as shown in FIG. 6, the output of the variable wavelength light source 112 has to be non-polarized wave employing the polarized scrambler 113.
The reason is that since output light of the variable wavelength light source is single polarized wave to encounter a problem in power stability and so forth, it becomes necessary to convert the output light into non-polarized wave light having higher power stability by means of the polarized wave scrambler. Accordingly, the polarized wave scrambler is required as additional hardware. Furthermore, construction and control mechanism for variation of wavelength of the variable wavelength light source 112 to make the device complicate.
It is an object of the present invention to provide a gain measurement device for an optical amplifier and measurement method therefore which permits continuous measurement at high speed and high precision with quite simple construction.
According to the first aspect of the present invention, a gain measurement device for measuring a gain versus wavelength characteristics of an optical amplifier, comprises:
a first light source outputting a light having a plurality of wavelengths for using in measurement of a plurality of wavelength points;
a second light source outputting a light having a plurality of wavelengths for using in measurement of a plurality of wavelength points other than the plurality of wavelength points;
means for selectively leading out one of the output of the first light source and a multiplexed output generated by multiplexing the output of the second light source with the output of the first light source; and
measuring means for deriving the gain versus wavelength characteristics of the optical amplifier on the basis of input and output characteristics when the output of the first light source is supplied to the optical amplifier and input and output characteristics when the multiplexed output is supplied to the optical amplifier.
In the preferred construction, the second light source includes a wide band light source outputting a light of wide band covering entire measurement band of the optical amplifier, and optical filtering means having a filtering characteristics for passing light of all measuring wavelength points other than the wavelength points to be measured in connection with the light of the first light source with taking the output of the wide band light source as input and blocking otherwise. The light filtering means may include a first arrayed-waveguide grating taking the output of the wideband light source as input to selectively demultiplex the light of entire measuring wavelength points other than the wavelength points to be measured in connection with the light of the first light source, and a second arrayed-waveguide grating multiplexing the lights demultiplexed by the first arrayed-waveguide grating. In the alternative, the optical filtering means may include a circulator taking the output of the wide band light source as input for a first port, and a plurality of fiber grating cascade connected sequentially to output from a second port of the circulator reflecting light of the entire measuring wavelength points other than the wavelength points to be measured in connection with the light of the first light source, and an output from a third port of the circulator is taken as output of the optical filtering means.
Also, the measuring means may includes a spectrum analyzer measuring spectra of input and output when the output of the first light source is supplied to the optical amplifier and measuring spectra of input and output when the multiplexed output is supplied to the optical amplifier, and means for measuring the gain versus wavelength characteristics on the basis of results of measurement by the spectrum analyzer.
Preferably, the gain measurement device may further comprise:
a variable optical attenuator controlling an input optical power to the optical amplifier;
an optical power meter measuring an input power to the optical amplifier; and
control means for controlling the variable optical attenuator on the basis of an output of the optical power meter so that the input optical power becomes a rated value.
According to the second aspect of the present invention, a gain measurement method for measuring a gain versus wavelength characteristics of an optical amplifier, comprises:
step of supplying a light having a plurality of wavelengths for using in measurement of a plurality of wavelength points from a first light source to an input of the optical amplifier;
step of multiplexing an output light from the first light source and an output light having a plurality of wavelengths from a second light source for using in measurement of wavelength points other than the wavelength points to be measured in connection with the light of the first light source to supply a multiplexed output to the input of the optical amplifier; and
measuring step of deriving the gain versus wavelength characteristics of the optical amplifier on the basis of input and output characteristics when the output of the first light source is supplied to the optical amplifier and input and output characteristics when the multiplexed output is supplied to the optical amplifier.
Preferably, the output light from the second light source maybe generated by operating a wide band light source for emitting a wide band light covering entire measurement band of the optical amplifier, and inputting the output of the wide band light source to optical filtering means having characteristics for passing light having entire measuring wavelength points other than the wavelength points to be measured in connection with the light of the first light source and blocking otherwise.
In the preferred process, the measuring step comprises:
step of measuring spectra of input and output when the output of the first light source is supplied to the optical amplifier and spectra of input and output when the multiplexed output is supplied to the optical amplifier; and
step of measuring the gain versus wavelength characteristics based on the results of measurement.
Also, the gain measurement method may further comprise:
step of measuring an input optical power to the optical amplifier by means of an optical power meter;
step of controlling the input optical power to the optical amplifier on the basis of an output of the optical power meter so that the input power to the optical amplifier becomes a rated value.
According to the third aspect of the present invention, a storage medium recording a program for implementing a gain measuring method for measuring a gain versus wavelength characteristics of an optical amplifier, the program comprises:
step of supplying a light having a plurality of wavelengths for using in measurement of a plurality of wavelength points from a first light source;
step of multiplexing an output light from the first light source and an output light having a plurality of wavelengths from a second light source for using in measurement of wavelength points other than the wavelength points to be measured in connection with the light of the first light source; and
measuring step of deriving the gain versus wavelength characteristics of the optical amplifier on the basis of input and output characteristics when the output of the first light source is supplied to the optical amplifier and input and output characteristics when the multiplexed output is supplied to the optical amplifier.
In the operation of the present invention, in addition to the multiple wavelength light source, the wavelength selective light source in which spontaneous emission light from the wide band light source is output after filtering through the optical filter so that outputs of the multiple wavelength light source becomes lights at wavelength points other than that of the multiple wavelength light source, provided separately from the spontaneous emission light (Pase) generated and amplified in the measurement objective optical amplifier, are provided. In case where the light (Pbase) of a plurality of wavelength points from the wavelength selective light source and the light from the multiple wavelength light source are superimposed with each other and in case of only light from the multiple wavelength light source, input and output spectra to and from the measurement objective optical amplifier are measured separately for performing measurement of gain versus wavelength characteristics of the measurement objective optical amplifier on the basis of the result of measurement of the input and output spectra for evaluation.